A series of novel murine recombinant viruses was isolated from natural and laboratory induced leukemias of mice. Each was genomically masked and biologically and biochemically unique. Natural defense mechanisms found in mouse sera were purified as lipid in nature. The spectrum of virus inactivation potential increased with purity of this oncornavirus inactivation factor (OIF). Seroimmunoprophylaxis experiments with anti MuLV gp70 IgG showed good protection of AKR mice if treatment occurred during a defined early period. Role of Eco and RM viruses was defined during the treatment period.